The present invention pertains to a ceramic cutting insert useful for the removal of material from a workpiece, e.g., chipforming machining of a workpiece. More specifically, the present invention pertains to a ceramic cutting insert useful for the removal of material from a workpiece, e.g., chipforming machining of a workpiece, wherein the ceramic cutting insert, which comprises a silicon nitride-based substrate or a SiAlON (silicon aluminum oxynitride)-based substrate, has a coating scheme thereon. The coating scheme includes an exposed alumina coating layer exhibiting a compressive stress.
Heretofore, artisans have used ceramic cutting inserts for the removal of material from a workpiece, e.g., chipforming machining of a workpiece. Patent documents provide some examples of these ceramic cutting inserts. Exemplary ceramics may comprise silicon nitride-based ceramics and SiAlON-based ceramics.
For example, U.S. Pat. No. 5,525,134 to Mehrotra et al. (assigned to Kennametal Inc.) discloses the use of a silicon nitride-based ceramic cutting insert. The silicon nitride-based ceramic cutting insert comprises at least about 85 volume percent beta silicon nitride and less than about 5 volume percent intergranular phase. Other additives include yttria and magnesia. The silicon nitride-based ceramic cutting insert may have a refractory coating thereon such as, for example, one or more of alumina, titanium carbide or titanium nitride.
U.S. Pat. No. 4,880,755 to Mehrotra et al. (assigned to Kennametal Inc.) discloses the use of a SiAlON-based ceramic cutting insert. The SiAlON-based ceramic cutting insert has a surface layer with an increased aluminum and oxygen content as compared to the bulk region. The process to make this cutting insert comprises the steps of preparing a SiAlON core composition, which may comprise alpha-prime-SiAlON and beta-prime SiAlON along with an intergranular phase. Next, the process comprises coating the SiAlON core with an alumina coating via CVD (chemical vapor deposition). Finally, the process comprises heat treating the SiAlON-alumina coating composite until a portion of the alumina diffuses into the SiAlON core. The SiAlON-based ceramic cutting insert, which has the alumina layer, may further include a refractory coating layer (e.g., carbides, nitrides, carbonitrides, oxides and mixtures thereof of vanadium, titanium, tantalum, niobium, hafnium or zirconium) over the alumina layer.
A coating scheme (i.e., coating layer(s)) applied via CVD to a cobalt cemented tungsten carbide substrate exhibits a tensile stress. The presence of the tensile stress is detrimental to the adhesion of the CVD coating scheme. Heretofore, some have used a post-coating blasting treatment to reduce the tensile stresses in the coating scheme or to change the stress to compressive stress in the coating scheme. As one example, U.S. Pat. No. 5,372,873 to Yoshimura et al. (assigned to Mitsubishi Materials Corporation) discloses shot peening the coating scheme on selected surfaces wherein the substrate is a cobalt cemented tungsten carbide, which exhibits a surface layer of cobalt enrichment. The result of shot peening is a change in the stress condition of the coating. In this regard, this patent shows that the shot peening process may be applied locally to parts of the cutting tool, for example to the rake surfaces, so that the residual tensile stresses in the primary coating thereon are lower than those tensile residual stresses in the primary coating on the flank surfaces of the cutting tool. Further, this patent also shows a shot peening treatment applied so that the residual stresses in the primary coating of the rake surfaces of the cutting tool are compressive, and that the residual stresses in the primary coating of the flank surfaces are tensile. As another example, U.S. Pat. No. 5,374,471 to Yoshimura et al. (assigned to Mitsubishi Materials Corporation) discloses shot peening a coating layer on a substrate wherein the shot peening is effective to treat only on the rake surfaces. The substrate is a cobalt cemented tungsten carbide, which exhibits a surface layer of cobalt enrichment.
A coating scheme (i.e., coating layer(s)) applied via CVD to a cermet substrate exhibits a tensile stress. United States Patent Application Publication No. US2006/0127671 A1 to Park et al. discloses the use of wet blasting as a post-coating treatment of a coating on a cemented carbide substrate, as well as cermet substrate. The wet blasting process uses a alumina particle, which have a size between 10-300 μm, water slurry. There is a reduction in the tensile stress or a conversion to compressive stress in the outer coating layer due to the wet blasting. Specific coating schemes include an alumina coating with titanium carbide/titanium nitride layer(s) on top of the alumina layer.
Another consequence of a post-coating treatment such as wet blasting is a smoothening of the coating surface. United States Patent Application Publication No. US2006/0204757 A1 to Ljungberg discloses a two-step wet blasting post-coating treatment for a coating scheme comprising a TiCxNy coating layer and an α-Al2O3 coating layer. The substrate is a cobalt cemented tungsten carbide material. These treatments show the affect due to variations in impingement angle and pressure. This treatment results in a smoother coating surface and a reduction in tensile stress or a conversion to compressive stress. U.S. Pat. No. 5,861,210 to Lenander et al., which discloses a Ti CxNyOz layer on alumina (see Col. 2, lines 30-43), discloses that it is known to vary the blasting parameters (see Example 1, Col. 4, line 48 through Col. 5, line 11) to achieve different results.
United States Patent Application Publication No. US2007/0009763 A1 to Littecke et al. discloses a wet blasting post-coating treatment for a coating scheme comprising a TiCxNy coating layer and an α-Al2O3 coating layer. The substrate is a cobalt cemented tungsten carbide material. This treatment results in a smoother coating surface and a low tensile stress TiCxNy coating layer and a smooth α-Al2O3 coating layer.
Each one of United States Patent Application Publication No. US2007/0298281 A1 to Andersson et al. and United States Patent Application Publication No. US2007/0298282 A1 to Andersson et al. discloses a wet blasting post-coating treatment for a coating scheme comprising a TiCxNy coating layer and an α-Al2O3 coating layer. The substrate is a cobalt cemented tungsten carbide material. This treatment results in a low tensile stress TiCxNy coating layer and a smooth α-Al2O3 coating layer. The stress condition is different between the rake surface and the flank surface.
In addition to wet blasting, some have used dry blasting to reduce the tensile stress in the coating. For example, U.S. Pat. No. 6,884,496 to Westphal et al. discloses the basic benefits of dry blasting a coated cutting insert wherein there is an increase in the compressive stress. See Col. 2, lines 42-67.
There should be an appreciation that some of the blasting operations remove only a small portion of the coating layer. For example, U.S. Pat. No. 7,531,213 to Bjormander pertains to a coated cutting tool insert wherein the post-treatment (preferably blasting or brushing) removes the outermost coating layer only on the edge-line and on the rake face.
Some of the above patent documents show a cutting insert useful for the removal of material from a workpiece, e.g., chipforming machining of a workpiece, wherein the coating scheme has been subjected to a post-coating treatment. This post-coating treatment has resulted in a reduction of the tensile stress or a conversion of the stress to compressive stress in the coating layer(s). This post-coating treatment has resulted in a smoothening of the outer coating layer.
However, many of these patent documents show a cutting insert, which possesses a cemented carbide substrate or in one case, a cermet substrate. These patent documents do not disclose the actual implementation of a post-coating treatment on a silicon nitride-based substrate or SiAlON-based substrate resulting in a reduction of the tensile stress or a conversion of the stress to compressive stress in the coating layer(s), as well as a smoothening of the outer coating layer.
In addition, some of these patent documents show the removal of only a portion of the outer coating layer. These patent documents do not disclose the actual implementation of a post-coating treatment on a silicon nitride-based substrate or SiAlON-based substrate leading to the complete removal of an outer coating layer. The result of the complete removal of the outer coating layer is a reduction of the tensile stress or a conversion of the stress to compressive stress in the coating layer(s) including the coating layer remaining as the outer coating layer.